Citizens will call the cops on you if they see you doing illegal activities. They will be more likely to call the cops on you if you have just hijacked their car.

To stop them from calling the cops on you, you have a couple of options. You can kill them, or you can take them out non-lethally. Taking out citizens that are trying to call the cops on you in a non-lethal manner is required for the "Communication Fail" achievement.

To do this, simply walk up to the citizen and press the button that pops up to perform a non-lethal melee takedown. You can also run at them and tackle them tot he ground to keep them from calling the police, or hack into their phone so they won't call the police either.

As you explore Chicago, you will sometimes get a notification that there is a crime in progress nearby. These are dynamic events. There's an infinite amount of them, but you have to complete at least 20 in order to get the achievement/trophy "Enforcer".

To start one of these missions, simply press the d-pad button that also appears when the crime in progress is announced to you.

There's an easy way to get away from a level 5 police chase. First, initiate the chase by attacking civilians and cops, and then mark the Bunker as your next destination on the map.

Drive to the Bunker and after you get across the bridge that connects the mainland of Chicago to the bunker island, be sure to hack into the bridge again so that the cops can't follow you with their cars anymore.

Go behind the crate that you use to get into the actual bunker. There are shipping containers here that you can get inside. The police helicopters will still pursue you, but if you position Aiden in the middle of one of these shipping containers, they won't be able to spot you. If the helicopter starts to position itself at a better angle in order to get a better look at you, go to the next shipping container over to continue avoiding detection.

Just wait inside these containers until the timer runs out on the police chase.